Secondary Beryl in Cordierite/Sekaninaite Pseudomorphs from Granitic Pegmatites – A Monitor of Elevated Content of Beryllium in the Precursor

Cordierite-group minerals (cordierite and sekaninaite) from granitic pegmatites are often strongly to completely altered to a fine- or coarse-grained mixture of muscovite, chlorite and/or, biotite, along with several less common secondary minerals, including mainly paragonite, tourmaline, and secondary beryl. The mixture is a common product of early subsolidus hydrothermal alteration at the examined pegmatites of the beryl-columbite subtype – Věžná I and Drahonín (Moldanubian Zone, Czech Republic) and Mount Begbie (Shuswap Complex, Canada); of the beryl-columbite-phosphate subtype – Szklary (Góry Sowie Block, Poland); and of miarolitic intragranitic pegmatites – Zimnik (Massif Strzegom-Sobótka, Poland). We studied in detail (EPMA, LA-ICP-MS) relics of primary cordierite/sekaninaite: Věžná I (Crd77–72Sek27–22MnCrd2–1, Be = 0.39–0.25 apfu, Li = 0.06–0.04 apfu), Drahonín (Crd13–9Sek74–71MnCrd17–16, Be = 0.24–0.18 apfu, Li = 0.07–0.05 apfu), Szklary (Crd50–49Sek30–26MnCrd25–21, Be = 0.45–0.41 apfu, Li ≤ 0.02 apfu), Mount Begbie (Crd34–33Sek53–43MnCrd24–14, Be = 0.33–0.29 apfu, Li = 0.26–0.23 apfu), and Zimnik (Crd2–1Sek75–71MnCrd28–23, Be = 0.25–0.15 apfu, Li = 0.18–0.12 apfu). Secondary beryl has a similar Mg/(Mg+Fe) ratio to its cordierite/sekaninaite precursor but is Mn depleted. The mineral assemblages and textures of the pseudomorphs were examined with a focus on secondary beryl, which forms anhedral grains to subhedral elongated crystals, up to 0.3 mm in size, or aggregates of these in textural equilibrium with associated phyllosilicates and tourmaline. Tourmaline is known from Věžná I, Drahonín, Mount Begbie, and Zimnik, the last also with topaz and “zinnwaldite” (a mineral with chemical composition between siderophyllite and polylithionite). Secondary beryl in pseudomorphs after cordierite/sekaninaite from granitic pegmatites and more evolved granites may have been often overlooked; hence, we present its textures and morphology so that it can be recognized during routine EPMA study and to study the source of elevated concentrations of Be in primary cordierite/sekaninaite. The empirical limit of detection of secondary beryl in pseudomorphs is ∼500–1000 ppm Be, which corresponds to ∼1–2 vol.% of secondary beryl. The chemical composition of the secondary beryl and other minerals indicate that the fluids responsible for the alteration were exsolved from the residual pegmatite melt and were not contaminated by fluids from the host rocks.

Genesis of felsic and mafic HP granulites from the Moldanubian Zone, Lower Austria

<p><span><span>The granulite occurrences from the Moldanubian zone were extensively studied in the last three decades and their metamorphic overprint at high pressures and at UHT conditions are well constrained. However, there are still some discrepancies regarding the prograde PT-path evolution, the genesis of the granulites and the tectonic processes required to produce the proposed PT-paths. Here we present a comprehensive petrological study where we have investigated more than 300 granulite samples from one of the largest occurrences, the Poechlarn-Wieselburg area - Dunkelsteinerwald. C</span><span>onventional geothermobarometry, garnet zoning pattern, thermodynamic modelling and Zr-in-rutile thermometry on rutile grains enclosed in garnets in felsic and mafic granulites allowed to constrain the prograde as well as the retrograde segments of the PT path. Polycrystalline melt inclusions and high-Ti biotite relics as well as a uniform temperature of approximately 800°C obtained from rutile inclusions (Zr-in-rutile thermometry) in garnet cores disagree with a continuous prograde garnet growth but favour a metastable overstepping of the garnet-in reaction and growth by the peritectic biotite breakdown reaction to garnet and melt within a very narrow PT interval. Subsequent heating to T>1000°C initiated a second stage of garnet growth with a very distinct chemical composition. The preservation of the zoning pattern at these metamorphic conditions clearly document a very short lived process. Diffusion models predict a time span of <5 Ma and cooling rates of 50-60°C/my.</span><span> Zircon U-Pb ages usually cluster around 340 Ma representing the metamorphic peak. However, in mafic granulites zircon ages from approximately 410 Ma to 340 Ma are obtained indicating either an older formation age for the precursor rock of the mafic granulites or just documenting the occurrence of xenocrysts. We applied a series of coupled petrological–thermomechanical tectono-magmatic numerical model to reproduce our deduced PTt-path that evolved from exhumation of subducted lower crust followed by intense heating at the crust-mantle boundary.</span></span></p>

Tectonic implications of the metamorphic field gradient in the Austrian Drosendorf and Gföhl units, Moldanubian Zone

<p>The Moldanubian Zone in Austria is traditionally subdivided into several tectonostratigraphic subunits, which were juxtaposed to their nowadays position during the Variscan orogeny. The Gföhl unit at the highest tectonic position exposes the Moldanubian granulites at the top, underlain by the granitic Gföhl orthogneiss. At its base lies the Raabs unit, a sequence of mafic rocks (amphibolites and sepentinites) accompanied by metasediments. The Drosendorf unit represents a sedimentary sequence mainly consisting of paragneisses, amphibolites and marbles. At the lowest position the Ostrong unit is dominated by low-P paragneisses with local appearances of eclogites.<br>A comprehensive study along four W–E profiles from the Danube valley (P1) in the south, to the Thaya valley (P4) in the north, revealed a disparate distribution of metamorphic conditions within the Drosendorf and the Gföhl units (Raabs unit and Gföhl orthogneiss). Along P1 several lithologies of the investigated units show similar P–T conditions of 0.8–1.2 GPa and 750–800 °C, followed by a decompression stage to 0.6–0.8 GPa and ~750 °C. Towards the north the temperature within the Drosendorf unit is continuously decreasing to 650–700 °C, at pressure conditions of 0.4–0.8 GPa. P–T conditions for Raabs unit and Gföhl orthogneiss are decreasing as well but are increasing again at P4. At the western end of P4 they reach similar conditions as in P1 (0.6–1.0 GPa and 725–800), but a decrease towards the east can be observed. A slight W–E decreasing trend is also observable in P2 and P3. Th–U–Pb microprobe dating of several metasedimentary and orthogneiss samples resulted in a Carboniferous age (~340 Ma) for metamorphism. At one locality in the south an older monazite generation indicates an incipient collisional metamorphism in the Devonian (~370 Ma).<br>The observed N–S gradient indicates that the southern parts represent formerly deeper buried lower crustal parts, whereas towards the north middle crustal levels are exposed, which were exhumed in a first stage. In a second stage of exhumation in the northernmost area, the oblique thrusting of lower crustal segment including the Gföhl unit onto the already exhumed lower-middle crustal parts caused the formation of a duplex structure, which is responsible for the present appearance of the area around the Drosendorf window.</p>

Axinit a doprovodné minerály z lokality Jezuitský rybník východně od Golčova Jeníkova (moldanubikum, Česká republika)

A new occurrence of axinite at the locality Jezuitský rybník near Sirákovice (ENE from Golčův Jeníkov), situated in rocks of the Variegated (Drosendorf) Series (Moldanubian Zone of the Bohemian Massif), is a nice example of contaminated pegmatite in a Ca-skarn with intense superimposed hydrothermal overprint. Axinite [axinite-(Fe) to axinite-(Mg)] forms young hydrothermal infill of pocket/fissure in pegmatite cutting a brecciated Ca-skarn. The hydrothermal assemblage includes amphibole II (actinolite to ferro-actinolite), albite, K-feldspar II, chlorite, epidote (locally containing 0.20 - 0.30 apfu REE), muscovite and Al,F-enriched titanite (with up to 2 % SnO2) passing exceptionally to unnamed CaAlFSiO4. Quartz, plagioclase (andesine), K-feldspar I and amphibole I (mostly K-rich or even potassian ferro-pargasite to ferro-tschermakite) originated in magmatic stage associated with intrusion of externally derived pegmatite melt. Sporadic garnet (grossular-rich almandine) represents relics of mineral assemblage of the host skarn. Dominance of Nd among REE in the REE-rich epidote is explained in terms of chemical fractionation of REE, probably caused by the presence of strong REE-complexing ligands (F-, OH- and/or CO32-) in aqueous fluids enriched in MREE/HREE due to alteration of garnet. With regard to the presence of B, Cr and elevated XMg in some hydrothermal phases compared to the older Fe-Mg minerals, we suggest circulation of fluids affecting host rocks as well as additional rock types.